The polymerization activity of a catalyst is a very important factor in any polymerization process. For a given catalyst system, the activity may depend on the polymerization conditions, such as temperature and pressure. However, once the polymerization conditions are set the activity depends strictly on the catalyst system. If the activity is not satisfactory, the amount of catalyst fed to the reactor and/or the residence time of the catalyst may be increased. However, this solution adversely affects plant operability from an economic standpoint, as an increase in catalyst feed means increased cost per unity of polymer produced, while increased residence time means lower productivity for the plant.
Ziegler-Natta catalyst systems are generally obtained by reacting an aluminum alkyl compound with a solid catalyst component comprising a magnesium halide and a titanium compound containing at least a Ti-halogen bond. As the catalyst component is responsible for both the activity and the polymer properties, once the catalyst system has been chosen for industrial production it may be exchanged for a different catalyst system having higher activity if the new system is capable of producing the desired polymer properties.
In ethylene polymerization processes, attempts to increase catalytic activity have previously been reported, including processes using of halogenated hydrocarbon compounds as activity enhancers as disclosed, for example, in U.S. Pat. Nos. 5,863,995, and 5,990,251. However, halogenated alkyls often demonstrate criticality when a given threshold amount is used, thereby generating a decrease in catalyst activity. In WIPO Pat. App. No. WO 2009/150111, halogenated alkyl compounds in which the halogens are linked to secondary carbon atoms are described that produce results similar to or better than the corresponding halogenated alkyls in which the halogen is linked to a primary hydrocarbon.
In some cases, the catalysts systems may contain an external donor added as a modifier of the polymer properties. For instance, U.S. Pat. No. 5,055,535 describes linear or cyclic monoethers used as external donors that narrow the molecular weight distribution and reduce catalyst activity.